1 .. SPDX-License-Identifier: GPL-2.0 2 3 Remote Controller devices 4 ------------------------- 5 6 Remote Controller core 7 ~~~~~~~~~~~~~~~~~~~~~~ 8 9 The remote controller core implements infrastructure to receive and send 10 remote controller keyboard keystrokes and mouse events. 11 12 Every time a key is pressed on a remote controller, a scan code is produced. 13 Also, on most hardware, keeping a key pressed for more than a few dozens of 14 milliseconds produce a repeat key event. That's somewhat similar to what 15 a normal keyboard or mouse is handled internally on Linux\ [#f1]_. So, the 16 remote controller core is implemented on the top of the linux input/evdev 17 interface. 18 19 .. [#f1] 20 21 The main difference is that, on keyboard events, the keyboard controller 22 produces one event for a key press and another one for key release. On 23 infrared-based remote controllers, there's no key release event. Instead, 24 an extra code is produced to indicate key repeats. 25 26 However, most of the remote controllers use infrared (IR) to transmit signals. 27 As there are several protocols used to modulate infrared signals, one 28 important part of the core is dedicated to adjust the driver and the core 29 system to support the infrared protocol used by the emitter. 30 31 The infrared transmission is done by blinking a infrared emitter using a 32 carrier. The carrier can be switched on or off by the IR transmitter 33 hardware. When the carrier is switched on, it is called *PULSE*. 34 When the carrier is switched off, it is called *SPACE*. 35 36 In other words, a typical IR transmission can be viewed as a sequence of 37 *PULSE* and *SPACE* events, each with a given duration. 38 39 The carrier parameters (frequency, duty cycle) and the intervals for 40 *PULSE* and *SPACE* events depend on the protocol. 41 For example, the NEC protocol uses a carrier of 38kHz, and transmissions 42 start with a 9ms *PULSE* and a 4.5ms SPACE. It then transmits 16 bits of 43 scan code, being 8 bits for address (usually it is a fixed number for a 44 given remote controller), followed by 8 bits of code. A bit "1" is modulated 45 with 560µs *PULSE* followed by 1690µs *SPACE* and a bit "0" is modulated 46 with 560µs *PULSE* followed by 560µs *SPACE*. 47 48 At receiver, a simple low-pass filter can be used to convert the received 49 signal in a sequence of *PULSE/SPACE* events, filtering out the carrier 50 frequency. Due to that, the receiver doesn't care about the carrier's 51 actual frequency parameters: all it has to do is to measure the amount 52 of time it receives *PULSE/SPACE* events. 53 So, a simple IR receiver hardware will just provide a sequence of timings 54 for those events to the Kernel. The drivers for hardware with such kind of 55 receivers are identified by ``RC_DRIVER_IR_RAW``, as defined by 56 :c:type:`rc_driver_type`\ [#f2]_. Other hardware come with a 57 microcontroller that decode the *PULSE/SPACE* sequence and return scan 58 codes to the Kernel. Such kind of receivers are identified 59 by ``RC_DRIVER_SCANCODE``. 60 61 .. [#f2] 62 63 The RC core also supports devices that have just IR emitters, 64 without any receivers. Right now, all such devices work only in 65 raw TX mode. Such kind of hardware is identified as 66 ``RC_DRIVER_IR_RAW_TX``. 67 68 When the RC core receives events produced by ``RC_DRIVER_IR_RAW`` IR 69 receivers, it needs to decode the IR protocol, in order to obtain the 70 corresponding scan code. The protocols supported by the RC core are 71 defined at enum :c:type:`rc_proto`. 72 73 When the RC code receives a scan code (either directly, by a driver 74 of the type ``RC_DRIVER_SCANCODE``, or via its IR decoders), it needs 75 to convert into a Linux input event code. This is done via a mapping 76 table. 77 78 The Kernel has support for mapping tables available on most media 79 devices. It also supports loading a table in runtime, via some 80 sysfs nodes. See the :ref:`RC userspace API <Remote_controllers_Intro>` 81 for more details. 82 83 Remote controller data structures and functions 84 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 85 86 .. kernel-doc:: include/media/rc-core.h 87 88 .. kernel-doc:: include/media/rc-map.h
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